1. Field of the Invention
The present invention generally relates to the fields of porous polymer materials and their biological uses. More specifically, it concerns particular 3-dimensional structural matrices containing nucleic acids, which provide nucleic acids with prolonged bioavailability and control over cellular migration, transfection and proliferation. The invention thus provides defined nucleic acid-matrix materials and methods of making and using such compositions, e.g., in cell transfection, gene expression and in vivo gene therapy, as exemplified by uses in wound healing and bone repair.
2. Description of Related Art
Lost or deficient tissue function leads to millions of surgical procedures each year and a loss to the western economies of hundreds of billions of dollars (Langer and Vacanti, 1993). Tissue engineering has emerged as a potential means of growing new tissues and organs to treat such patients, and several approaches are currently under investigation to engineer structural tissues.
Improved biodegradable polymers and copolymers have recently been generated for use in the tissue engineering field. This has allowed developments in the generation of autologous and allogeneic tissues intended for use in transplantation. The role of biomaterials in the in vitro expansion of cultured cells is generally to serve as a vehicle to localize the cells of interest. Biomaterials can also be used in vivo to deliver biologically active substances.
Biodegradable homopolymers and copolymers of lactic and glycolic acid, poly(lactic-co-glycolic acid) (PLGA; now also termed poly(lactide-co-glycolide) or PLG), have become attractive candidates for fabricating tissue engineering matrices due to their flexible and well defined physical properties and relative biocompatibility. The degradation products of these polymers are also natural metabolites and are readily removed from the body.
Several techniques have been used to fabricate polymers into porous matrices for tissue engineering applications, including solvent-casting/particulate leaching (SC/PL) (Mikos et al., 1994); phase separation (Lo et al., 1995); fiber extrusion and fabric forming processing (Cavallaso et al., 1994); and gas foaming (Mooney et al., 1996). However, the current techniques each suffer from their particular drawbacks.
The solvent-casting/particulate leaching and phase separation approaches require the use of organic solvents. Residues of organic solvents that remain in these polymers after processing may damage transplanted cells and nearby tissue and/or inactivate biologically active factors incorporated into the polymer matrix for controlled release. Fiber forming typically requires high temperatures (above the transition temperature of polymer), and is not amenable to processing amorphous polymers. The high temperatures used in such processes would likely denature any biologically active molecules incorporated into the matrix.
The gas foaming method, as exemplified by Mooney et al. (1996), provides a technique to fabricate highly porous matrices from PLGA using a high pressure gas that avoids the use of organic solvents and high temperatures. However, the technique typically yields a closed pore structure, which is disadvantageous in many applications of cell transplantation. In addition, a solid skin of polymer results on the exterior surface of the foamed matrix and this may lead to mass transport limitations.
Therefore, there exists in the art a need for improved polymer materials for use in tissue engineering and in vivo protocols. In terms of in vivo, rather than in vitro uses, several other problems also need to be overcome. These include limitations such as providing biological materials to an appropriate site of the body, exposure of the materials to the appropriate cell types, efficient release and/or provision of the biological materials, maintenance of an effective concentration of biological materials, and prolonged and appropriate activity of the biological materials.